An old example of organic luminescence device is, e.g., one using luminescence of a vacuum-deposited anthracene film (Thin Solid Films, 94 (1982) 171). In recent years, however, in view of the advantages such as easiness of providing a large-area device compared with an inorganic luminescence device and a possibility of obtaining desired luminescence colors in view of the development of various new materials and drivability at low voltages, an extensive study related to forming a luminescence device of a high-speed responsiveness and a high efficiency has been conducted.
As described in detail in, e.g., Macromol. Symp. 125, 1-48 (1997), an organic EL device generally has a structure comprising upper and lower electrodes and a plurality of organic film layers between the electrodes formed on a transparent substrate. Basic structures thereof are shown in FIGS. 1(a) and (b).
As shown in FIG. 1, an organic EL device generally has a structure comprising a transparent electrode 14, a metal electrode 11, and a plurality of organic film layers therebetween on a transparent substrate 15.
In the device of FIG. 1(a), the organic layers comprise a luminescence layer 12 and a hole-transporting layer 13. For the transparent electrode 14, ITO, etc., having a large work function are used, for providing a good hole-injection characteristic from the transparent electrode 14 to the hole-transporting layer 13. For the metal electrode 11, a metal, such as aluminum, magnesium or an alloy of these, having a small work function is used for providing a good electron-injection characteristic. These electrodes have a thickness of 50-200 nm.
For the luminescence layer 12, aluminum quinolynol complexes (a representative example thereof is Alq3 shown hereinafter), etc., having an electron-transporting characteristic and luminescence characteristic are used. For the hole-transporting layer, biphenyldiamine derivatives (a representative example thereof is .alpha.-NPD shown hereinafter), etc., having an electron-donative characteristic are used.
The above-structured device has a rectifying characteristic, and when an electric field is applied between the metal electrode 11 as a cathode and the transparent electrode 14 as an anode, electrons are injected from the metal electrode 11 into the luminescence layer 12 and holes are injected from the transparent electrode 15. The injected holes and electrons are recombined within the luminescence layer 12 to form excitons and cause luminescence. At this time, the hole-transporting layer 13 functions as an electron-blocking layer to increase the recombination efficiency at a boundary between the luminescence layer 12 and hole-transporting layer 13, thereby increasing the luminescence efficiency.
Further, in the structure of FIG. 1(b), an electron-transporting layer 16 is disposed between the metal electrode 11 and the luminescence layer 12. By separating the luminescence and the electron and hole-transportation to provide a more effective carrier blocking structure, effective luminescence can be performed. For the electron-transporting layer 16, an electron-transporting material, such as an oxadiazole derivative, is used.
Known luminescence processes used heretofore in organic EL devices include utilizing an excited singlet state and utilizing an excited triplet state, and the transition from the former state to the ground state is called “fluorescence” and the transition from the latter state to the ground state is called “phosphorescence”. And the substances in these excited states are called a singlet exciton and a triplet exciton, respectively.
In most of the organic luminescence devices studied heretofore, fluorescence caused by the transition from the excited singlet state to the ground state has been utilized. On the other hand, in recent years, devices utilizing phosphorescence via triplet excitons have been studied.
Representative published literature may include:                Article 1: Improved energy transfer in electrophosphorescent device (D. F. O'Brien, et al., Applied Physics Letters, Vol. 74, No. 3, p. 422 (1999)); and        Article 2: Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo, et al., Applied Physics Letters, Vol. 75, No. 1, p. 4 (1999)).        
In these articles, a structure including 4 organic layers devices as shown in FIG. 1(c) has been principally used, including, from the anode side, a hole-transporting layer 13, a luminescence layer 12, an exciton diffusion-prevention layer 17 and an electron-transporting layer 11. Materials used therein include carrier-transporting materials and phosphorescent materials, of which the names and structures are shown below together with their abbreviations.                Alq3: aluminum quinolinol complex        α-NPD: N4,N4′-di-naphthalene-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine        CBP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline        PtOEP: platinum-octaethylporphyrin complex        Ir(ppy)3: iridium-phenylpyridine complex         
The above-mentioned Articles 1 and 2 both have reported structures as exhibiting a high efficiency, including a hole-transporting layer 13 comprising α-NPD, an electron-transporting layer 16 comprising Alq3, an exciton diffusion-preventing layer 17 comprising BCP, and a luminescence layer 12 comprising CBP as a host and ca. 6% of platinum-octaethylporphyrin complex (PtOEP) or iridium-phenylpyrimidine complex (Ir(ppy)3) as a phosphorescent material dispersed in mixture therein.
Such a phosphorescent material is particularly noted at present because it is expected to provide a high luminescence efficiency in principle for the following reasons. More specifically, excitons formed by a carrier recombination comprise singlet excitons and triplet excitons in a probability ratio of 1:3. Conventional organic EL devices have utilized fluorescence of which the luminescence efficiency is limited to at most 25%. On the other hand if phosphorescence generated from triplet excitons is utilized, an efficiency of at least three times is expected, and even an efficiency of 100%, i.e., four times, can be expected in principle, if a transition owing to the intersystem crossing from a singlet state having a higher energy to a triplet state is taken into account.
However, like a fluorescent-type device, such an organic luminescence device utilizing phosphorescence is generally required to be further improved regarding the deterioration of luminescence efficiency and device stability.
The reason for the deterioration has not been fully clarified, but the present inventors consider it to be as follows based on the mechanism of phosphorescence.
In the case where the luminescence layer comprises a host material having a carrier-transporting function and a phosphorescent guest material, a process of phosphorescence via triplet excitons may include unit processes as follows:                1. transportation of electrons and holes within a luminescence layer,        2. formation of host excitons,        3. excitation energy transfer between host molecules,        4. excitation energy transfer from the host to the guest,        5. formation of guest triplet excitons, and        6. transition of the guest triplet excitons to the ground state and phosphorescence.        
Desirable energy transfer in each unit process and luminescence are caused in competition with various energy deactivation processes.
Particularly, in a phosphorescent material, this may be attributable to a life of the triplet excitons, which is longer by three or more digits than the life of a singlet exciton. More specifically, because it is held in a high-energy excited state for a longer period, it is likely to react with surrounding materials and cause polymer formation among the excitons, thus incurring a higher probability of a deactivation process resulting in a material change or life deterioration, as we have considered.
Needless to say, a luminescence efficiency of an organic luminescence device is increased by increasing the luminescence quantum yield of a luminescence center material, but it is also an important factor for enhancing the luminescence intensity of the device to increase the concentration of a luminescence material in the luminescence layer.
The luminescence intensity is increased in proportion to the concentration of a luminescence material in a luminescence layer in the case of a low concentration (up to several wt. %) of the luminescence material in the luminescence layer. However, above several % or 7%, a deviation from the proportional relationship is observed, and the luminescence intensity is rather lowered to result in a worse efficiency. This phenomenon is reported in Japanese Laid-Open Patent Application (JP-A) 05-078655, JP-A 05-320633, etc., and is known as concentration extinction or concentration deactivation.
Actually, in the case of using Ir(ppy)3 in CBP as the host material, the best luminescence efficiency is attained at a concentration of ca. 6-7%, and the luminescence efficiency is rather lowered thereabove, down to about a half at 12% concentration and 1/10 or below at 100% concentration (Applied Physics Letters 4, vol. 75, 1999).
The phenomenon is caused by abundant presence of molecules in the triplet excited state waiting for luminescence in the case of a phosphorescence substance having a life of triplet exciton longer by 3 digits or more than the life of the singlet exciton. In this state, thermal deactivation of losing energy due to a mutual interaction of triplet excitons is likely to occur. This is called triplet-triplet extinction and is associated with a lowering in luminescence efficiency at a high current density. Further, it is also considered that due to a long retention time at a high energy state, the excitons have an increased probability of reacting with a surrounding material and forming polymers of excitons, thereby causing deactivation, or even leading to a material change or a deterioration of life.